The present invention relates to the detection of watermarks embedded in a frame of content data and, more particularly, the present invention relates to methods and/or apparatuses for detecting a watermark that are resistant to resizing, rotation, and/or translation.
It is desirable to the publishers of content data, such as music, video, software, and combinations thereof to prevent or deter the pirating of the content data. The use of watermarks has become a popular way of thwarting pirates. A watermark is a set of data containing a hidden message that is embedded in the content data and stored with the content data on a storage medium, such as a digital video disc (DVD), compact disc (CD), read only memory (ROM), random access memory (RAM), magnetic media, etc. The hidden message of the “embedded watermark” is typically a copy control message, such as “do not copy” or “copy only once.”
When a quantum of data comprising the content data and the embedded watermark is correlated with a reference watermark, a determination can be made as to whether the embedded watermark is substantially similar to, or the same as, the reference watermark. If a high correlation exists, then it may be assumed that the message of the embedded watermark corresponds to a message of the reference watermark. For example, the quantum of data may be a frame of data, such as video data, in which pixel data of the frame of video data has been embedded with a watermark (“the embedded watermark”). Assuming that the frame of data has not been distorted in some way, when a reference watermark that is substantially the same as the embedded watermark is correlated with the frame of video data, a relatively high output is obtained. This is so because a one-for-one correspondence (or registration) between the data of the embedded watermark and the data of the reference watermark will tend to increase a correlation computation. Conversely, if the embedded watermark contained in the frame of video data has been altered in a way that reduces the one-for-one correspondence between the embedded watermark and the reference watermark, the correlation will yield a relatively low result.
Often, the correlation computation involves performing a sum-of-products of the data contained in the frame of data and the data of the reference watermark. Assuming that the frame of data and the reference watermark include both positive magnitude values and negative magnitude values, the sum-of-products will be relatively high when the data of the embedded watermark aligns, one-for-one, with the data of the reference watermark. Conversely, the sum-of-products will be relatively low when the data of the embedded watermark does not align with the reference watermark.
A data detector, such as a standard correlation detector or matched filter, may be used to detect the presence of an embedded watermark in a frame of content data, such as video data, audio data, etc. The original or reference position of the embedded watermark is implicitly determined by the design of the hardware and/or software associated with the detector. These types of correlation detectors are dependent upon specific registration (i.e., alignment) of the embedded watermark and the reference watermark.
Pirates seeking to wrongfully copy content data containing an embedded watermark (e.g., one that proscribes copying via a hidden message: “do not copy”) can bypass the embedded watermark by distorting the registration (or alignment) between the embedded watermark and the reference watermark. By way of example, a frame of content data containing an embedded watermark may be slightly rotated, resized, and/or translated from an expected position to a position that would prevent a one-for-one correspondence (perfect registration) between the embedded watermark and the reference watermark. Editing and copying equipment may be employed to achieve such distortion.
The malfeasance of pirates notwithstanding, inadvertent distortion of the embedded watermark may occur during the normal processing of the content data (containing an embedded watermark) in a computer system or consumer device. For example, the content data (and embedded watermark) of a DVD may be inadvertently distorted while undergoing a formatting process, e.g., that converts the content data from the European PAL TV system to the US NTSC TV system, or vice versa. Alternatively, the content data and embedded watermark may be distorted through other types of formatting processes, such as changing the format from a wide-screen movie format to a television format. Indeed, such processing may inadvertently resize, rotate, and/or translate the content data and, by extension, the embedded watermark, rendering the embedded watermark difficult to detect.
Different types of watermark systems exist that purport to be robust to resizing and translation. One such watermark system typically embeds the watermark in a way that is mathematically invariant to resizing and translation, such as the system disclosed in U.S. Pat. No. 6,282,300, the entire disclosure of which is hereby incorporated by reference. The detector used in this type of system does not have to adjust to changes in the position and/or size of the embedded watermark. Such a system is typically based on Fourier-Mellin transforms and log-polar coordinates. One drawback of this system is that it requires complex mathematics and a particularly structured embedded watermark pattern and detector. This system cannot be used with pre-existing watermarking systems.
Another prior art watermark system uses repetitive watermark blocks, wherein all embedded watermark blocks are identical. The watermark block in this type of system is typically large and designed to carry the entire copy-control message. The repetition of the same block makes it possible to estimate any resizing of the embedded watermark by correlating different portions of the watermarked image and finding the spacing between certain positions. The resizing is then inverted and the reference block is correlated with the adjusted image to find the embedded watermark and its position simultaneously. An example of this system is the Philips VIVA/JAWS+ watermarking system. A disadvantage of this system is that the design of the embedded watermark must be spatially periodic, which does not always occur in an arbitrary watermarking system.
Yet another type of watermarking system includes an embedded template or helper pattern along with the embedded watermark in the content data. The detector is designed to recognize the reference location, size and shape of the template. The detector attempts to detect the template and then uses the detected position of the template to estimate the actual location and size of the embedded watermark. The system then reverses any geometric alterations so that the correlation detector can detect and interpret the embedded watermark. This system is disadvantageous, however, since the templates tend to be fragile and easily attacked.
Accordingly, there is a need in the art for a new method and/or system for detecting an embedded watermark in a frame of data that is robust despite rotation, resizing and/or translation of the embedded watermark as compared to a reference watermark.